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He said the latest figures on sharks, collated by scientists for a UN conference on migratory species this week, mirrored previous studies.

"Ecologists have made some of these calculations about the value of ecosystem services and the value of clean natural environments," Wing said. "When you put them in dollar terms, they're huge."

Duffy said savvy eco-tourism operators had sprung up in Gisborne and the Poor Knights Islands to give people shark experiences, including free-diving with sharks.

"It's increasingly popular in New Zealand. It doesn't have to be in a cage." Duffy said overseas research showed that when big shark numbers dropped, smaller sharks thrived, feeding on some fish popular with recreational fishers.

In New Zealand, overfishing of the gummy shark - the "fish and chips shark" - was blamed as the reason for paddle crabs overrunning swimming beaches.